This is my CHE 130 webpage and I support sections 103 and 104. I will be posting handouts and study tips periodically throughout the quarter so check them out once they are posted! Chemistry can be difficult but it can also be awesome! If you have any questions come to an SI session or leave a comment below. You can also upload any of your helpful resources in the comment section!
I am a junior majoring in Chemistry with a Medical/Biochemistry concentration. I enjoy the sciences because it allows me to have a deeper understanding to how and why the world works the way it does. As part of my passion for chemistry, I research with Dr. Grice in McGowan South during the school year. Outside of academics I enjoy eating food, listening to music, drinking coffee/tea, and exploring new places in Chicago when I have the time. If you have suggestions, absolutely let me know!
Empirical Formula Problem Based on Percent Composition
Your professor has sent off a compound to be examined and gets results in the form of percent composition. The elements contained in the compound are C, H, O, and N. The results are:
- C: 64.69%
- H: 5.92%
- O: 15.67%
- N: (There’s a printer error and this is missing)
Based on these results, what is the empirical formula of this compound?
First things first, we want to know as much as possible in order to get so we want to find the percent composition of nitrogen. This is done below. (NOTE: Significant figures!)
So now you have all the percentages which are kind of messy to deal with, as they are, so we have to play around with them to get useful information. In this case, we multiply them by 100 grams to get masses of each element as seen below.
64.69% Carbon×100g/(100%)=64.69g Carbon
5.92% Hydrogen×100g/(100%)=5.92g Hydrogen
15.67% Oxygen×100g/(100%)=15.67g Oxygen
13.72% Nitrogen×100g/(100%)=13.72g Nitrogen
Much more manageable! When you see masses for different elements, you can’t really compare them as they are. They have different molecular masses after all! So the conversion to moles is required as seen below. (NOTE: Conversions made using molecular mass listed in you periodic table)
64.69g Carbon×(1 mole)/(12.0107 g)=5.386 mole Carbon
5.92gHydrogen×(1 mole)/(1.00794 g)=5.87 mole Hydrogen
15.67g Oxygen×(1 mole)/(15.9994 g)=0.9794 mole Oxygen
13.72g Nitrogen×(1 mole)/(14.00674 g)=0.9495 mole Nitrogen
So know that we have all these moles, it’s time to do a bunch of molar ratios. It’s typical to measure the molar ratios using the smallest molar number as the denominator. In this case, nitrogen looks the smallest so we’ll use that value! Calculations are shown below. (NOTE: You can use another molar number but it’s more work and who wants that?)
(0.9495 mole Nitrogen)/(0.9495 mole Nitrogen)=1mole N
(0.9794 mole Oxygen)/(0.9495 mole Nitrogen)=1.031≈1 mole O
(5.87 mole Hydrogen)/(0.9495 mole Nitrogen)=6.18≈6 mole H
(5.386 mole Carbon)/(0.9495 mole Nitrogen)=5.672≈5.5 mole C
So we can’t actually solve the empirical formula yet. As far as I know, no company has ever promoted a molecule that has half of an atom involved (although that would be cool). So we now have to multiply the equation in such a way to get a whole number. We can use 2 as shown below.
2(C_5.5 H_6 O_1 N_1 )=C_11 H_12 O_2 N_2
So the empirical formula (and consequently molecular formula) of our compound is C11H12O2N2. This is coincidentally the molecular formula of a compound known as tryptophan. This is an important amino acid that is also often blamed for making you tired around Thanksgiving when you eat too much turkey! If you got the same result, congratulations!
Exam II Worksheet
These are example problems that should reflect material regarding the Rydberg equation, empirical mass, and molecular mass. The last question is an application of what you learned incorporated with stoichiometric ratios. Good Luck!
1) a) 94.9nm 1) b) ni=4 1) c) nf=4 2) a) C2H3O3 2) b) C4H6O6 3) a) CHN 3) b) C3H3N3 4) 55.84g -> Fe
Hybridization of Elements
For each of the carbons labelled, list the hybridization.
All are sp3 with exception to C1 which is sp2. The secret is to count the groups around each element you are examining. C5, for instance, has 2 carbons as well as the H and OH around it. Having 4 groups tells us that this carbon is sp3 hybridized.
Note: For the curious, this is a Fisher Projection of D-Glucose. For those who are planning on taking Organic Chemistry, this will certainly come up at some point!
Exam III Worksheet
Example problems are included. No answer key though.
Identify the Element
If you have a compounds XCl2 and Ca(OH)2 combining to form X(OH)2 and CaCl2, what is the identity of X if you used 25.74g of X(OH)2 and obtain 30.00g of CaCl2 at the end of the reaction? (Assume 100% yield)
Exam IV Worksheet
Example problems are included. No answer key though.
Extra Reduction-Oxidation Problems
Do the first three categories. [ilink url=”http://www.chemistry.wustl.edu/~coursedev/Online%20tutorials/Plink/Redox/redoxset.htm”]LINK[/ilink]
Exam I Study Tips
- Always watch out for significant figures. Even if it’s only a point lost for each problem, it can mean the difference between a B+ or an A- for and exam grade for instance.
- For conversions, always pay attention to what’s given. I find that just writing out what’s given to you and what you need to find concentrates the block of text into something more manageable!
- In knowing which conversion factor to use, I find it helpful to imagine that the unit you want is always on the top of a conversion factor and the unit you want to eliminate always on the bottom of a conversion factor.
- When you do calculations, it can be useful to write out each number you get on the exam paper. If the number doesn’t feel right, you can do the calculation again and double check. Note: Circle the right answer so your professor knows which answer is the one you want evaluated
- Be prepared to explain important concepts. There is generally a question that expects you to explain the concept in detail.
- It’s easy to be discouraged by the BIG problems. With high-point values usually associated with them, this is understandable. Do not be discouraged, however. Even if you tackle the problem step-by-step as far as you can with correct significant figures and units, that’s still a lot of potential partial credit!
Exam II Study Tips
- Study all of the big math problems. I would way this includes stuff involved with calculating molar mass, empirical formulas, molecular formulas, and the Rydberg equation. While this is not the alpha and omega of the exam, it could be >40% of the exam I imagine.
- Be able to explain various aspects of the periodic trends and NOT JUST remember what they look.
- Practice writing the electron configurations and produce drawings based on this information if required.
- Study, study, study the charges of different ions. This will help so much with naming ionic compounds!
Exam III Study Tips
- Study the conceptual problems (hybridization and MO theory) as these are worth decent amount of points on the exam and tend to sneak up on students
- Percent yield can be a concept that many students unfortunately ignore. DON’T do that!
- When doing molecular modeling, you can always practice drawing structures when doing chemical equation balancing and limiting reactant problems.
- This exam appears to be very evenly distributed this time around so it pays to study all areas rather than just a few problem types.
Exam IV Study Tips
- Study Solubility Rules. While generally NOT recommended when compared to more reliable methods, memorization is probably the best way given the current timeframe.
- Study Acids/Bases. Understand, generally, what is formed and what such reactants generally look like.
- Study Reduction/Oxidation. Remember that both occur simultaneously with a loss and gain of electrons occurring between compounds/elements.
- Be able to incorporate the above concepts in previous sorts of problems. Much of what you learned in terms of general reactions types and concepts can be applied mathematically in different ways. Albeit, maybe it’s applied in solution concentration, limiting reactant, and identify-the-unknown type problems.
Final Exam Study Tips
- Go through all your past exams and assignments. Do you still remember how to approach certain problems? It’s essential to review concepts that you have not seen in a while.
- Do the all of the previous and practice exams. Grade yourself and be honest. How confident do you feel about the material? If you don’t feel solid about the material, go over it again!
- There’s a review session being held on Tuesday, November 17, 2015 from 2-4PM in McGowan South 108. To help the SI team gauge the concepts that students feel uncomfortable with, there’s a survey at [https://docs.google.com/forms/d/13bjJ68HO2R828g4C_N6UPCIQJeC0Du4A-_d6z5_WA7A/viewform?usp=send_form]
Videos and Handouts
Significant Figures, Conversions, and Atomic Mass Calculation
Wonderful video from the Khan Academy showing conversions in action!
[ilink url=”https://www.youtube.com/watch?v=ScvuRb6vsz4″]Khan Academy Conversions[/ilink]
Also, since you need to know a decent portion of the periodic table of the elements, here is an updated version of the elements song. ASAP Science rocks you guys!
[ilink url=”https://www.youtube.com/watch?v=zUDDiWtFtEM”]ASAP Science Elements Song[/ilink]
For anyone who wants a quick review of moles, here’s a video by Ted-Ed.
[ilink url=”https://www.youtube.com/watch?v=TEl4jeETVmg”]TED-ED Moles[/ilink]
A video on electron orbitals from CrashCourse
[ilink url=”https://www.youtube.com/watch?v=cPDptc0wUYI”]Electron Orbitals[/ilink]
How to Draw 3-D Structures (By Mikayla Patella-Buckley)
MO Theory Series (By Ben’s Chem Videos)
Additional Online Resources
Significant Figures Rules